This invention relates to analog to digital converters and particularly to a programmable analog to digital converter that provides manually selected transfer functions or time varying transfer functions between the analog input signal and the digital output signal.
In some systems such as ultrasonic or radiography systems for medical diagnosis it would be desirable to be able to emphasize certain gray levels without requiring an excessive number of finite gray levels to be provided in the overall system design. For example, in a converter system which has 16 gray levels and a 4 bit digital output, the sensitivity could be increased in certain input voltage amplitude level regions so that the system in the area of interest would be equivalent to a system having a much greater number of gray levels. In an ultrasonic system for example, the encoding of the full range of the image may be normally sufficient with 16 digital levels. However, a band of information in the lower version of amplitudes may need encoding to the equivalent of 32 levels full scale in order to separate specific anatomical differences in the echo returns. Utilizing the system of the invention, 8 main levels might be distributed between levels 2 and 4 of the more gross 16 level system giving the desired resolution but using only 16 levels overall. This arrangement would result in a saving of approximately 20% in any memory which followed the A/D converter as well as a saving of approximately 50% of the number of components in the A/D converter itself. Also, in systems such as ultrasound or radiography systems, the echo signal or input video signal is applied and reflected from different body structures such as the liver and kidneys which have characteristics such that the reflected signals from each has different amplitude characteristics. In order to emphasize and clearly image all of the body structures it would be desirable if matching analog to digital transfer functions were provided for each body structure. Thus, an analog to digital converter that is programmable so that different transfer functions could be selected or could be automatically provided for different body structures would be highly useful in a great number of systems such as for example, ultrasonic or radiography.
U.S. Pat. Nos. 3,544,779 and 3,646,548 teach arrangements for providing or controlling the reference voltages applied to the comparators in an analog to digital converter. The U.S. Pat. No. 3,544,779 teaches a self adjusting radix converter and the U.S. Pat. No. 3,646,548 teaches the use of nonlinear comparators having a fixed nonlinear curve, but neither patent teaches the use of a programmed input source to provide desired transfer functions. A programmable analog to digital converter controlling the pulse rate and sensing the number of output pulses is described in U.S. Pat. No. 3,349,390 but this converter only converts the voltage output of a nonlinear transducer into a pulse group proportional in number to the input signal by utilizing a programmed pulse generator. The digital system of this U.S. Pat. No. 3,349,390 does not teach programming of a ladder network type analog to digital converter.